紫外线是一种电磁辐射，可使黑光海报发光，并导致夏季晒黑和晒伤。然而，过多地暴露于紫外线辐射会损害活组织。 电磁辐射来自太阳，以不同波长和频率的波或粒子形式传播。这种广泛的波长范围被称为电磁 (EM) 光谱。光谱一般按波长递减、能量和频率递增的顺序分为七个区域。常见的名称是无线电波、微波、红外线(IR)、可见光、紫外线 (UV)、X 射线和伽马射线。
紫外线 (UV) 光落在可见光和 X 射线之间的 EM 光谱范围内。它的频率约为每秒8 × 10 14至 3 × 10 16周期或赫兹 (Hz)，波长约为 380 纳米（1.5 × 10 -5英寸）至约 10 纳米（4 × 10 -7英寸） . 根据美国海军的《紫外线辐射指南》，紫外线一般分为三个子波段：
UVB 或中紫外 (280–315 nm)
UVC 或远紫外线 (180–280 nm)
该指南继续指出，“波长从 10 nm 到 180 nm 的辐射有时被称为真空或极端紫外线。” 这些波长被空气阻挡，只能在真空中传播。
紫外线辐射有足够的能量来破坏化学键。由于它们具有更高的能量，UV 光子会引起电离，即电子从原子中脱离的过程。由此产生的空位会影响原子的化学性质，并导致它们形成或破坏它们原本不会形成的化学键。这可能对化学处理有用，或者可能对材料和活组织造成损害。这种损害可能是有益的，例如，在xiaodu表面时，但它也可能是有害的，特别是对皮肤和眼睛，它们受到高能 UVB 和 UVC 辐射的不利影响更大。
人们遇到的大部分自然紫外线来自太阳。然而，根据毒理学计划(NTP)的数据，只有大约 10% 的阳光是紫外线，其中只有大约三分之一会穿透大气层到达地面。在到达赤道的太阳紫外线能量中，95% 是 UVA，5% 是 UVB。来自太阳辐射的可测量 UVC 无法到达地球表面，因为高层大气中的臭氧、分子氧和水蒸气完全吸取了最短的紫外线波长。尽管如此，根据 NTP 的“第 13 次致癌物报告”，“广谱紫外线辐射 [UVA 和 UVB] 是最强且对生物危害更大的” 。
晒黑是对暴露于有害 UVB 射线的反应。从本质上讲，晒黑是由身体的自然防御机制启动的结果。它由一种叫做黑色素的色素组成，它是由皮肤中的黑色素细胞产生的。黑色素吸取紫外线并将其作为热量消散。当身体感觉到阳光的伤害时，它会将黑色素发送到周围的细胞中，并试图保护它们免受更多的伤害。色素会导致皮肤变黑。
“黑色素是一种天然防晒剂，”塔夫茨大学医学院皮肤病学助理教授 Gary Chuang 在 2013 年的一次采访中告诉 Live Science。然而，持续暴露于紫外线辐射会压倒身体的防御能力。发生这种情况时，会发生毒性反应，导致晒伤。紫外线会破坏人体细胞中的 DNA。身体感知到这种破坏并用血液充斥该区域以帮助愈合过程。疼痛的炎症也会发生。通常在过度晒太阳的半天之内，晒伤的特征性红龙虾外观就会开始显现，并感觉到。
有时，带有因太阳光线而发生突变的 DNA 的细胞会变成问题细胞，它们不会死亡，但会像癌症一样继续增殖。“紫外线会在 DNA 和 DNA 修复过程中造成随机损伤，从而使细胞获得避免死亡的能力，”Chuang 说。
根据内布拉斯加大学的说法，在用于照明的荧光灯管中，“当电流通过汞蒸气时，会产生波长为 254 nm 的紫外线以及发出的蓝光” 。“这种紫外线辐射是不可见的，但比发出的可见光包含更多的能量。紫外线的能量被荧光灯内部的荧光涂层吸取，并以可见光的形式重新发射。” 没有相同荧光涂层的类似管会发出可用于xiaodu表面的紫外线，因为紫外线辐射的电离效应可以杀死大多数细菌。
黑光灯管通常使用汞蒸气来产生长波 UVA 光，这会导致某些染料和颜料发出荧光。玻璃管上涂有深紫色滤光材料，可阻挡大部分可见光，使荧光辉光显得更加明显。xiaodu等应用不需要此过滤。
除了太阳之外，还有许多天体紫外线辐射源。据美国宇航局称，非常大的年轻恒星发出的大部分光都是紫外波长。由于地球的大气层阻挡了大部分紫外线辐射，特别是在较短波长的情况下，因此使用高空气球和轨道望远镜进行观测，这些望远镜配备了专门的成像传感器和滤光片，用于在 EM 光谱的紫外线区域进行观察。
根据密苏里州立大学天文学教授罗伯特·帕特森的说法，大多数观测都是使用电荷耦合器件 (CCD) 进行的，这种检测器旨在对短波长光子敏感。这些观测可以确定最热恒星的表面温度，并揭示地球和类星体之间存在干预气体云。
据英国癌症研究中心称，虽然暴露在紫外线下会导致皮肤癌，但某些皮肤状况可以使用紫外线进行治疗。在称为补骨脂素紫外线治疗 (PUVA) 的程序中，患者服用药物或涂抹乳液以使皮肤对光敏感。然后紫外线照射在皮肤上。PUVA 用于治疗淋巴瘤、湿疹、牛皮癣和白癜风。
最近的研究表明，紫外线可能在地球上生命的起源，尤其是 RNA 的起源中发挥了关键作用。在 2017 年《天体物理学杂志》的一篇文章中，该研究的编辑指出，红矮星可能不会发出足够的紫外线来启动形成核糖核酸所需的生物过程，而核糖核酸对地球上所有形式的生命都是必需的。该研究还表明，这一发现可能有助于在宇宙其他地方寻找生命。
What Is Ultraviolet Light?
Ultraviolet light is a type of electromagnetic radiation that makes black-light posters glow, and is responsible for summer tans — and sunburns. However, too much exposure to UV radiation is damaging to living tissue.
Electromagnetic radiation comes from the sun and transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic (EM) spectrum. The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves,microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.
Ultraviolet (UV) light falls in the range of the EM spectrum between visible light and X-rays. It has frequencies of about 8 × 1014 to 3 × 1016 cycles per second, or hertz (Hz), and wavelengths of about 380 nanometers (1.5 × 10?5 inches) to about 10 nm (4 × 10?7 inches). According to the U.S. Navy's "Ultraviolet Radiation Guide," UV is generally divided into three sub-bands:
UVA, or near UV (315–400 nm)
UVB, or middle UV (280–315 nm)
UVC, or far UV (180–280 nm)
The guide goes on to state, "Radiations with wavelengths from 10 nm to 180 nm are sometimes referred to as vacuum or extreme UV." These wavelengths are blocked by air, and they only propagate in a vacuum.
UV radiation has enough energy to break chemical bonds. Due to their higher energies, UV photons can cause ionization, a process in which electrons break away from atoms. The resulting vacancy affects the chemical properties of the atoms and causes them to form or break chemical bonds that they otherwise would not. This can be useful for chemical processing, or it can be damaging to materials and living tissues. This damage can be beneficial, for instance, in disinfecting surfaces, but it can also be harmful, particularly to skin and eyes, which are most adversely affected by higher-energy UVB and UVC radiation.
Most of the natural UV light people encounter comes from the sun. However, only about 10 percent of sunlight is UV, and only about one-third of this penetrates the atmosphere to reach the ground, according to the National Toxicology Program (NTP). Of the solar UV energy that reaches the equator, 95 percent is UVA and 5 percent is UVB. No measurable UVC from solar radiation reaches the Earth's surface, because ozone, molecular oxygen and water vapor in the upper atmosphere completely absorb the shortest UV wavelengths. Still, "broad-spectrum ultraviolet radiation [UVA and UVB] is the strongest and most damaging to living things," according to the NTP's "13th Report on Carcinogens."
A suntan is a reaction to exposure to harmful UVB rays. Essentially, a suntan results from the body's natural defense mechanism kicking in. This consists of a pigment called melanin, which is produced by cells in the skin called melanocytes. Melanin absorbs UV light and dissipates it as heat. When the body senses sun damage, it sends melanin into surrounding cells and tries to protect them from sustaining more damage. The pigment causes the skin to darken.
"Melanin is a natural sunscreen," Gary Chuang, an assistant professor of dermatology at Tufts University School of Medicine, told Live Science in a 2013 interview. However, continued exposure to UV radiation can overwhelm the body's defenses. When this happens, a toxic reaction occurs, resulting in sunburn. UV rays can damage the DNA in the body's cells. The body senses this destruction and floods the area with blood to help with the healing process. Painful inflammation occurs as well. Usually within half a day of overindulging in the sun, the characteristic red-lobster look of a sunburn begins to make itself known, and felt.
Sometimes the cells with DNA mutated by the sun's rays turn into problem cells that don't die but keep proliferating as cancers. "The UV light causes random damages in the DNA and DNA repair process such that cells acquire the ability to avoid dying," said Chuang.
The result is skin cancer, the most common form of cancer in the United States. People who get sunburned repeatedly are at much higher risk. The risk for the deadliest form of skin cancer, called melanoma, doubles for someone who has received five or more sunburns, according to the Skin Cancer Foundation.
Other UV sources
A number of artificial sources have been devised for producing UV radiation. According to the Health Physics Society, "Artificial sources include tanning booths, black lights, curing lamps, germicidal lamps, mercury vapor lamps, halogen lights, high-intensity discharge lamps, fluorescent and incandescent sources, and some types of lasers."
One of the most common ways of producing UV light is passing an electric current through vaporized mercury or some other gas. This type of lamp is commonly used in tanning booths and for disinfecting surfaces. The lamps are also used in black lights that cause fluorescent paints and dyes to glow. Light-emitting diodes (LEDs), lasers and arc lamps are also available as UV sources with various wavelengths for industrial, medical and research applications.
Many substances — including minerals, plants, fungi and microbes, as well as organic and inorganic chemicals — can absorb UV radiation. Absorption causes electrons in the material to jump to a higher energy level. These electrons can then return to a lower energy level in a series of smaller steps, emitting a portion of their absorbed energy as visible light. Materials used as pigments in paint or dye that exhibit such fluorescence appear brighter under sunlight because they absorb invisible UV light and re-emit it at visible wavelengths. For this reason they are commonly used for signs, safety vests and other applications in which high visibility is important.
Fluorescence can also be used to locate and identify certain minerals and organic materials. According to Thermo Fisher Scientific, Life Technologies, "Fluorescent probes enable researchers to detect particular components of complex biomolecular assemblies, such as live cells, with exquisite sensitivity and selectivity."
In fluorescence flashlight used for lighting, "ultraviolet radiation with a wavelength of 254 nm is produced along with the blue light that is emitted when an electric current is passed through mercury vapor," according to the University of Nebraska. "This ultraviolet radiation is invisible but contains more energy than the visible light emitted. The energy from the ultraviolet light is absorbed by the fluorescent coating inside the fluorescent lamp and re-emitted as visible light." Similar tubes without the same fluorescent coating emit UV light that can be used to disinfect surfaces, since the ionizing effects of UV radiation can kill most bacteria.
Black-light tubes typically use mercury vapor to produce long-wave UVA light, which causes certain dyes and pigments to fluoresce. The glass tube is coated with a dark-purple filter material to block most of the visible light, making the fluorescent glow appear more pronounced. This filtering is not needed for applications such as disinfecting.
Besides the sun, there are numerous celestial sources of UV radiation. Very large young stars shine most of their light in ultraviolet wavelengths, according to NASA. Because Earth's atmosphere blocks much of this UV radiation, particularly at shorter wavelengths, observations are conducted using high-altitude balloons and orbiting telescopes equipped with specialized imaging sensors and filters for observing in the UV region of the EM spectrum.
According to Robert Patterson, a professor of astronomy at Missouri State University, most observations are conducted using charge-coupled devices (CCD), detectors designed to be sensitive to short-wavelength photons. These observations can determine the surface temperatures of the hottest stars and reveal the presence of intervening gas clouds between the Earth and quasars.
While exposure to UV light can lead to skin cancer, some skin conditions can be treated using UV light, according to Cancer Research UK. In a procedure called psoralen ultraviolet light treatment (PUVA), patients take a drug or apply a lotion to make their skin sensitive to light. Then a UV light is shone on the skin. PUVA is used to treat lymphoma, eczema, psoriasis and vitiligo.
It may seem counterintuitive to treat skin cancer with the same thing that caused it, but PUVA can be useful due to UV light’s effect on the production of skin cells. It slows down the growth that plays a major role in the disease’s development.
Key to the origin of life?
Recent research suggests that UV light may have played a key role in the origin of life on Earth, especially the origin of RNA. In a 2017 article in the Astrophysics Journal, the authors of the study note that red dwarf stars may not emit enough UV light to start the biological processes needed for the formation of ribonucleic acid, which is necessary for all forms of life on Earth. The study also suggests this finding could help in the search for life elsewhere in the universe.
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